AJN^D PHYSIOLOGY OF THE SPONGIAH^. 
783 
In the large open areas of the skeleton of Euplectella aspergillum, Owen, the hexradiate 
forms, ranging from fig. 24 to fig. 33, Phil. Trans. 1858, are exceedingly abundant, and a 
considerable number of them are not developed to the extent of the full number of their 
radii. This may probably arise from the development of the radii being stimulated by 
the necessities of the mass of sarcodous tissues in which they are imbedded, and conse- 
quently where no necessity for their presence exists they would not be put forth. In 
the trifurcate and quadrifurcate hexradiate forms, if we may judge from the termination 
of their radii, they, like the simple stellate forms, are either purely consolidating, or they 
combine with that office that of defensive spicula also, as far as regards the sarcodous 
substance in which they are imbedded. 
AVe can scarcely imagine any defensive properties in the slender and complicated but 
elegant forms of the fioricomo-stellate spicula, and it is probable that their office is purely 
that of assisting in the consolidation of the sarcodous substance. 
The whole of these beautiful stellate forms of spicula are siliceous, while their homo- 
logues in the Gorgoniadse and^the compound Tunicata are calcareous ; and it is somewhat 
remarkable that hitherto none of these forms have been found in the calcareous species 
of sponges. 
Spicula of the Ovaria and Gemmules. 
We find the same laws in force regarding the spicula in the structure of the minute 
bodies which have beeffidesignated gemmules by previous writers on the Spongiadee, that 
obtain in the sponges themselves. In some they serve the purposes of internal skeleton 
and defensive spicula as well. In others they combine the offices of tension and defen- 
sive organs, and frequently they are very different in form from those of the parent 
sponge. In the first part of this paper, in Plate XXVI. figs. II to 42, Phil. Trans. 1858, 
I have figured the varieties of form that I have hitherto found in the ovaria and gem- 
mules, and I have shown that these bodies may be classed in three groups. 
1. Those which have the spicula disposed at right angles to lines radiating from the 
centre of the ovarium to its surface. 
2. Spicula disposed in lines radiating from the centre to the circumference of the 
ovarium. 
3. Spicula disposed in fasciculi in the substance of the gemmule from the centre to 
the circumference. 
In Spongilla Carteri, Boweebank, MS., and S. fuviatilis, Johnston, our commonest 
British species, belonging to the first group, the external series of spicula of the ovaria are 
of the same form as those of the skeleton, but frequently somewhat shorter. They are 
disposed irregularly over the surface of the ovarium, and firmly cemented to it by the 
middle of the shaft, while each of their apices is projected in tangental lines. Thus 
their shafts perform the office of tension spicula, while their terminations become efficient 
weapons of defence. Fig. 11, Plate XXVI. Phil. Trans. 1858, represents the spiculum 
of the ovarium of S. Carteri. 
In other cases in this group we find these spicula differing from those of the skeleton 
